Media access control (mac) for an active rfid system

ABSTRACT

A system of radio frequency communication between Readers and Tags having a scalable Media Access Control (MAC) protocol for an active RFID system employing multiple access. The system comprising at least one tag Reader and a plurality of Tags each having a unique tag ID and deployed in a region in which at least some of the Tags are in radio communication with the tag Reader. A multi-dimensional addressing scheme of the MAC protocol enables division of the plurality of Tags into a number of groups for different stages of MAC protocol processing, based on different portions of the tag ID. The MAC has an inherent flexibility in the sense that it allows readers to communicate with different versions of tags and also optimize communication parameters to reader&#39;s capabilities, without pre-configuration of the tags. Furthermore, the MAC supports means to achieve very high access reliability, such as relay between tags.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/018,228, filed Jan. 23, 2008, the contents of which is incorporatedherein in its entirety by reference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to radio frequency Tags-Readers forcommunication, identification and distance measurement and, inparticular, it concerns a Media Access Control (MAC) for a system ofradio frequency communication between Readers and Tags employingmultiple access.

The need for RFID system grows in many areas and for a large diversityof applications, such as asset management and tracking, process control,and asset visibility. It is expected that the use of active RFID deviceswill reach mass deployments in the future; however at the current time,some limiting factors still exist, among them tag prices, and also someperformance limitations. One of the major factors that affects systemperformance, is the MAC (or the “air interface” protocol).

Prior art MAC schemes include the IEEE 802.11 standard that forms thebasis of the WiFi networking which is used all over the world, as wellas the IEEE 802.15.4 standard that forms the basis of ZigBee devices.There are also numerous proprietary communication protocols.

The IEEE 802.11 MAC scheme is basically suited for file transfers andnot for short packet transfer. While 802.11 offers a very good solutionfor connection to the Internet for “high-end” mobile devices, such aslaptop computers, it is not optimized for very low end devices with hardbattery life, cost and size constrains. It does not seem to be the rightchoice for a scalable active RFID system.

The IEEE 802.15.4 MAC scheme defines two types of Tag devices, namelyReduced Function Devices (RFDs) arid Full Function Devices (FFDs), andassumes that devices alternate between “on” and “off” modes. Theoriginator of messages is the RFD. This is one of the reasons that suchscheme is not ideal for RFID applications. Another concern is that thesystem supports Mesh connectivity only between FFDs, while RFDs canconnect only to a FFD. Yet another important consideration is that mostZigBee applications deal with a small to medium number of networkeddevices (e.g. for home control or for automatic meter reading). Further,the 802.15.4 multiple access scheme has a fixed structure.

Radio Frequency IDentification (RFID) is a method of storing andremotely retrieving data using devices called RFID Tags. An RFID Tag isa small device that can be attached to a physical object, animal, orperson. Each tag has a unique tag identification number (tag ID). RFIDTags receive and respond to radio-frequency queries from an RFID Readeror from neighboring Tags. An RFID system includes several componentsincluding mobile Tags, Tag Readers, and application software. Thisinvention relates to a system using Active Tags (that is, Tags that havean internal power source and can receive and transmit RF signals). Dueto the need to obtain a long life span for an active Tag that employs asmall battery, the Tag typically alternates between “sleep” and “active”modes. It may typically be in the “active” mode 2% to 10% of the time.

The RFID system enables a query to be received by the mobile Tag and theTag responds with data. The data is received by an RFID Reader andprocessed according to the needs of a particular application. The datatransmitted by the Tag may provide identification or locationinformation, or specifics about the product tagged, such as type, serialnumber, price, color, date of purchase.

U.S. Pat. No. 6,992,986, “Integrated self-optimizing multi-parameter andmulti-variable point to multipoint communication system” to Reza et al.,discloses an adaptive system for point to multipoint wirelesscommunication. The Phy and MAC layers include an adaptive set ofparameters. However, in this application, the base station adjustscommunication with each customer premises equipment individually,according to the latter capabilities, link condition, etc.; while in thecurrent disclosure, the reader sends to the tag population some MACparameters according to the reader's capabilities and to the assumedgroup size in each request, it is not required for tag to remember thoseparameters and it enables the tag to communicate with different readersaccordingly.

U.S. Pat. No. 7,257,095, “Power saving via physical layer addressfiltering in WLANs” to Liu, describes a system and method for savingpower in a wireless network, using a physical layer address filteringprotocol based on a partial address subset of the complete destinationMAC address.

Several patents relate to the usage of relay communication. U.S. Pat.No. 6,785,510 “Routing in a multi-station network” to Larsen, teaches amethod of relaying data between mobile stations in a cellularcommunication system. It is based on a broadcast control channel, aswell as on a calling channel (or channels), by which the mobile stationobtain connectivity information. This is very different from the systemand method of relay that is disclosed in the current disclosure.

U.S. Pat. No. 6,501,582, “Cascaded line-of-sight free-spacecommunication system” to Chiou et al., teaches an optical communicationsystem where repeaters are used in order to relay data between stations.

U.S. Pat. No. 6,987,770 “Frame forwarding in an adaptive network” toYonge, discloses a scheme which uses payload encapsulation and controlinformation, to allow a frame originated from a first station, andintended for a second station, to be forwarded by an intermediatestation. The first station selects the intermediate station based on thedata transmission quality that is expected to be achieved.

RFID systems are generally used for a multitude of asset managementapplications. A warehouse, or a logistic center, may include a largenumber of tagged objects—say, thousands to hundreds of thousands. Insuch a case there is more than one reader on site, and possibly tens ofreaders are employed to cover the whole volume. Therefore, it isnecessary to “think networking” namely, to design the communicationprocess and protocol so as to allow efficient access to Tags whiletaking into account several basic factors and requirements:

-   -   (a) The system may include a very large number of tagged objects        but should be also efficient if only a small number of Tags is        present.    -   (b) Tags are typically low-cost devices, with limited        functionality.    -   (c) Objects dynamically enter and/or leave the customer        premises.    -   (d) Tags alternate between on and off (or active & sleep) modes.    -   (e) The main mode of communication is between Readers and Tags.        In several useful RFID applications, the Reader (also called        “Interrogator” in the RFID literature) is the originator of the        session.    -   (f) Having stated the above, the system needs to support some        manner of Tag to Tag communication.    -   (g) The Reader, when acting as a collector node, should be able        to maintain a large aggregate data rate, keeping the response        time for Tag querying low.    -   (h) The overall system should be able to support many tagged        objects communicating simultaneously.    -   (i) Unicast, Multicast and Broadcast modes should be supported,        as these modes are useful in various workflows required by        customers' operations.    -   (j) The MAC should be “power aware”, that is to say able to        operate with Tags that alternate between on and off (or active &        sleep) modes (even with a low duty cycle).    -   (k) The MAC should support the basic application needs; the        latter include Read tag data, Write tag data, and locate a        tagged object.    -   (l) The communication reliability of the overall system, that is        to say communication between the Readers and all Tags currently        in the system at any given time, should be very high.

There is therefore a need for a Media Access Control (MAC) for a systemof radio frequency communication between Readers and Tags that addressmost if not all of the above listed requirements.

SUMMARY OF THE INVENTION

The present invention is a Media Access Control (MAC) for a system ofradio frequency communication between Readers and Tags employingmultiple access.

According to the teachings of the present invention there is provided, asystem of radio frequency communication between Readers and Tags, thesystem comprising; a) at least one tag Reader; b) a plurality of Tagsdeployed in a region in which at least some of the Tags are in radiocommunication with the tag Reader, and each tag is given a unique tagID, and c) a Media Access Control protocol such that the tag Reader isconfigured to send and receive radio communication to and from at leastsome of the plurality of Tags, the plurality of Tags are configured tosend and receive radio communication to and from the tag Reader and theMedia Access Control protocol is configured to control the radiocommunication between the tag Reader and the plurality of Tags, and theMedia Access Control protocol includes a multi-dimensional addressingscheme allowing Readers to efficiently address tag communities rangingfrom small to very large in unicast, multicast and broadcast modes, themulti-dimensional addressing scheme enables division of the plurality ofTags into a number of groups for different stages of Media AccessControl protocol processing, based on different portions of the tag ID.

According to a further teaching of the present invention, a hashfunction is used to transform the tag ID into a pseudo random ID, so asto achieve better spreading of tag population and lower statisticaldependency between dimensions.

According to a further teaching of the present invention, a portion ofthe tag ID is used for message screening in the PHY level and therebyavoid a need for at least one tag in the plurality of Tags to fullyprocess a Media Access Control protocol portion of the radiocommunication.

According to a further teaching of the present invention, a portion ofthe tag ID is used for setting the on/off timing of at least one tag inthe plurality of Tags.

According to a further teaching of the present invention, a portion ofthe ID is used by at least one tag to choose one reply window out of aset of possible reply windows.

According to a further teaching of the present invention, a portion ofthe ID is used by the tag Reader for addressing an acknowledgement to atleast one Tag of the plurality of Tags that the tag Reader received aradio communication from the at least one Tag.

There is also provided according to the teachings of the presentinvention, a system of radio frequency communication between Readers andTags, the system comprising; a) at least one tag Reader; b) a pluralityof Tags deployed in a region in which at least some of the Tags are inradio communication with the tag Reader, at least some tags are in radiocommunication with other Tags in the plurality of Tags and each tag isgiven a unique tag ID, and c) a Media Access Control protocol such thatthe tag Reader is configured to send and receive radio communication toand from at least some of the plurality of Tags, the plurality of Tagsare configured to send and receive radio communication to and from thetag Reader and other Tags in the plurality of Tags and the Media AccessControl protocol is configured to control the radio communicationbetween the tag Reader and the plurality of Tags, such that the MediaAccess Control protocol uses a combination of a session ID,acknowledgements and use of other tags as relays to achieve highreliability of radio communication, in broadcast, multicast and unicastmodes.

According to a further teaching of the present invention, a portion ofthe tag ID is used by the tag Reader to indicate to at least one tag ofthe plurality of Tags that the tag Reader received a the radiocommunication from that particular tag.

According to a further teaching of the present invention, the tag Readerinitiates a relay session after a rate of direct replies from theplurality of tags falls below a predetermined threshold.

According to a further teaching of the present invention, each broadcastmessage is given a session ID, and at least one tag of the plurality oftags replies to radio communications having a particular session IDuntil the at least one tag receives an acknowledgement radiocommunication having the same session ID.

There is also provided according to the teachings of the presentinvention, a system of radio frequency communication between Readers andTags, the system comprising; a) at least one tag Reader; b) a pluralityof Tags deployed in a region in which at least some of the Tags are inradio communication with the tag Reader, at least some tags are in radiocommunication with other Tags in the plurality of Tags and c) a MediaAccess Control protocol such that the tag Reader is configured to sendand receive radio communication to and from at least some of theplurality of Tags, the plurality of Tags are configured to send andreceive radio communication to and from the tag Reader and other Tags inthe plurality of Tags and the Media Access Control protocol isconfigured to control the radio communication between the tag Reader andthe plurality of Tags; wherein the Media Access Control protocolincludes tag reply parameters in each the radio communication so as toenable tags to respond to the tag Reader in an optimal manner for thattag Reader's capabilities.

According to a further teaching of the present invention, the radiocommunication includes an indication of the payload starting point, andeach generation of tag Readers and/or Tags decode that portion of theMedia Access Control protocol with which it is familiar, ignores thatportion of the Media Access Control protocol with which it is unfamiliarand jumps to the payload start point, and the radio communicationincludes a block of variable length, such that old generation Tagsforward the block without decoding, and new generation tag Readersand/or Tags decode and modify the block as needed.

There is also provided according to the teachings of the presentinvention, a system of radio frequency communication between Readers andTags, the system comprising; a) at least one tag Reader; b) a pluralityof Tags deployed in a region in which at least some of the Tags are inradio communication with the tag Reader, and c) a Media Access Controlprotocol such that the tag Reader is configured to send and receiveradio communication to and from at least some of the plurality of Tags,the plurality of Tags are configured to send and receive radiocommunication to and from the tag Reader and the Media Access Controlprotocol is configured to control the radio communication between thetag Reader and the plurality of Tags; wherein the radio communicationincludes an indication of a payload start point, and each generation oftag Readers and/or Tags decode that portion of the Media Access Controlprotocol with which it is familiar, ignores that portion of the MediaAccess Control protocol with which it is unfamiliar and jumps to thepayload start point, allowing reader to efficiently communicate with amixture of tag generations and/or tag versions.

There is also provided according to the teachings of the presentinvention, a system of radio frequency communication between Readers andTags, the system comprising; a) at least one tag Reader; b) a pluralityof Tags deployed in a region in which at least some of the Tags are inradio communication with the tag Reader, at least some tags are in radiocommunication with other Tags in the plurality of Tags and c) a MediaAccess Control protocol such that the tag Reader is configured to sendand receive radio communication to and from at least some of theplurality of Tags, the plurality of Tags are configured to send andreceive radio communication to and from the tag Reader and other Tags inthe plurality of Tags and the Media Access Control protocol isconfigured to control the radio communication between the tag Reader andthe plurality of Tags; wherein the radio communication includes a blockof variable length, such that old generation Tags forward the blockwithout decoding, and new generation tag Readers and/or Tags decode andmodify the block as needed.

According to a further teaching of the present invention, the radiocommunication includes an indication of payload start point, and eachgeneration of tag Readers and/or Tags decode that portion of the MediaAccess Control protocol with which it is familiar, ignores that portionof the Media Access Control protocol with which it is unfamiliar andjumps to the payload start point, allowing reader to efficientlycommunicate with a mixture of tag generations and/or tag versions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a system of radio frequencycommunication between Readers and Tags, constructed and operationalaccording to the teachings of the present invention;

FIG. 2 is a schematic illustration of a Media Access Control protocolconfigured and operational according to the teachings of the presentinvention;

FIG. 3 is a schematic graphical illustration of an exemplary method forsetting Tag IDs and separating Tags into sub-group types according tothe teachings of the present invention;

FIG. 4 is a schematic block diagram of Downlink and Uplink communicationaccording to the teachings of the present invention;

FIG. 5 is a schematic illustration of “relay” communication betweenReaders and Tags is a system of radio frequency communication betweenReaders and Tags, constructed and operational according to the teachingsof the present invention; and

FIG. 6 is a schematic representation of “relay” communication in thesystem of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a Media Access Control (MAC) for a system ofradio frequency communication between Readers and Tags employingmultiple access.

The principles and operation of a Media Access Control according to thepresent invention may be better understood with reference to thedrawings and the accompanying description.

By way of introduction, in a system of radio frequency communicationemploying multiple access, it is possible to make use of the inherentcapacity of devices which allow for multiple return transmissions from aplurality of Tags to a single Reader, possibly leading to a substantialthroughput. However since the Tags should be low-complexity devices,there typically is no synchronization between them. Therefore, in theTag-to-Reader direction the MAC follows basically the rules of ALOHA,with multiple time frequency or code slots. Such a MAC scheme is knownin the art and reference is now made to two previous patentapplications, PCT International Patent Application Publication Nos.WO2003/098528 and WO2006/030422, both of which are incorporated byreference for all purposes as if fully set forth herein.

PCT International Patent Application Publication No. WO2003/098528, (PCTPatent Application No. PCT/IL2003/00358), entitled “Method and systemfor distance determination of RF Tags” discloses an RFID system havingthe capability of automatically identifying unknown Tags by sending abroadcast interrogation message signal and receiving responses from allTags that receive the message signal.

PCT International Patent Application Publication No. WO2006/030422entitled (PCT Patent Application No. PCT/IL2005/000967), “Communicationand Distance Measurement in an addressed Wide Band RFID System”, dealswith Unicast modes of communication, both Reader to Tag and Tag toReader, as well as a Relay mode in which the Reader communicates with aTag through an “intermediate” Tag.

These applications relate mainly to the application possibilities usingwide band Tags, employing novel means of communication and air-interfaceof an Impulse Radio, which allows, in principle, for simultaneousresponse messages from a plurality of Tags to a single Reader's querymessage. Collision resolution in an ALOHA-type multiple access scheme,as disclosed in the aforementioned patent applications, forms a basis ofthe present invention. It should be noted that the term “message” isused herein to refer to a radio communication in either directionbetween a tag reader and at least one tag, and between a tag and one ormore other tags. Therefore, the terms “message” and “radiocommunication” (when used as a noun) are used interchangeably herein.

Therefore, MAC of the present invention provides a method ofcommunication between at least one tag Reader and a community of,preferably Active, Tags, in which the number of tags is dynamic andchanging as tagged items are moved within, as well as into and out of,the monitored region. The MAC of the present invention self adjusts tothe number of Tags in the community and is equally proficient with bothsmall and very large communities of Tags. As used herein, the terms“small community” and “small number of Tags” generally refers to a Tagcommunity ranging from several Tags to several tens of Tags. As usedherein, the terms “very large community” and “very large number of Tags”generally refers to a Tag community ranging from several tens ofthousands of Tags to more than several hundreds of thousands of Tags. Itwill be appreciated, therefore, that a Tag community according to thepresent invention includes the full range from only a few Tags tohundreds of thousands of Tags.

Referring now to the drawings, FIG. 1 schematically illustrates asystem, generally referred to as 2, composed of one or more Readers 4and unknown number of Tags 6. The number of Tag may range for few dozensto X00,000. Readers and Tags communicate using wireless multiple accessprotocol allowing up to N stations to be received by a Readersimultaneously. Readers provide the “gateway” between the Tags and theInformation Technology (IT) infrastructure, or between Tags and theuser. In some cases the readers connect to an Edge Server that connectsto the upper layer (that is, to the customer's IT infrastructure). Thesystem may also contain a mixture of high-end Readers and low-endReaders, namely readers with a large capability as a collector node (interms of aggregate rate supported) and reader with lower capabilities(supporting a lower number of simultaneous tag transmissions).

Some Tags 6 a communicate with at least one Reader 4, while other,“hidden” Tags 6 b are located such that they cannot communicate with anyof the Readers 4 in the system due to link budget limitations. It willbe appreciated that Readers may be hardwired to the IT infrastructure orcommunicate with the IT infrastructure via wireless technology asillustrated by Reader 4 a is FIG. 1.

In order to meet the requirements listed in the Field and Backgroundsection above, a combination of prior methods and new inventions isused. The following are features of the scaleable novel MAC scheme ofthe present invention.

A sophisticated addressing allowing Readers to efficiently address tagcommunities ranging from small to very large in unicast, multicast andbroadcast modes, while allowing for the fluctuation in the number ofTags inherent if a dynamic tag community such as, but not limited to awarehouse, shipping yard or retail outlet.

Use of PHY address screening so as to allow the tag to screen nonrelevant messages after PHY header is decoded, based on thesophisticated addressing mentioned above, and reduce the amount of timethe tag needs to be turned on for MAC address processing hence reducingpower consumption.

High reliability so as to support a very large community of Tags withvery high (99.9999%) reliability by ensuring communication with nearlyall Tags in a X00,000 Tag environment.

Flexibility so as to allow Readers to communicate with new (incoming tothe coverage area) Tags without having to first download the currentsystem parameters and configuration, yet optimizing the Tags response tothe current Reader configuration. Furthermore, flexibility to make Tags“future ready”, that is, provide future compatibility with newgenerations of Tags without the need to update Tag software.

Sophisticated Addressing

The novel MAC protocol of the present invention allows easy addressingof both small and very big Tag communities.

Given a limited capacity of the uplink (Tags to Readers communication)capacity of the system, the addressing of a Reader to Tags broadcastshould be done so the number of replies to a Reader will be optimized tothe uplink multiple access capacity. Therefore, when addressing a smallcommunity of Tags, the broadcast addressing should divide the Tagcommunity to a small number of groups. Similarly, when addressing a verylarge Tag community, the addressing should divide the Tag community to alarge number of groups.

The system may work in “simple access mode” or in “beacon access mode”.Similar to other MAC methods, in beacon mode the Reader transmits abeacon message from time to time allowing the Tags to synchronize on theReader timing. In simple mode, the Reader transmits only when it hasdata, and the Tags are not synchronized with the Reader timing. FIG. 2shows Tag replies in a beacon scheme. The period between beacons isdivided to frames 20 and each frame 20 is divided to slots 22.

Broadcast or multicast addressing is done according with the followingprinciples:

A Tag ID is used to divide the Tag community to groups such as, bynon-limiting example, use of four of the ID bits, divides the tagcommunity into 16 tag groups.

While Tag ID is usually generated according to logistics andmanufacturing guidelines, the use of a hash function spreads the Tag IDsto uniformly spread numbers where RANDOM_ID_Hash Function {ID}.Different parts of the RANDOM_ID will be used in order to divide the tagcommunity into different MAC stages. It is preferable that the hashfunction make these stages (denoted for example as x, y and z) asorthogonal as possible, with minimal statistical dependency. That is tosay, a hash function is used to transform the tag ID into a pseudorandom ID, so as to achieve better spreading of tag population and lowerstatistical dependency between parameter dimensions.

For broadcast only, a random number could have been used by the Tag toset its membership in each of the sub-group types detailed below.However, the use of a known hash function ID allows the Readers to knowexactly how to address a specific Tag when unicast or multicastaddressing is desired. The use of a hash function has not been used inany previous MAC protocol.

The RANDOM_ID is used to separate Tags into sub-group types in severaldimensions, as illustrated in FIG. 3.

“On-off time” 30 level sets each Tag “on” time into a beacon slot 20 (1to N), as seen in FIG. 2.

Physical (PHY) addressing level sets the “hardware” addressing 32 (1 toM) for each Tag 6 in the community. The use of PHY address screeningsaves tag power by letting the tag screen non relevant messages in thePHY stage without needing to stay awake (power on) to process the fullMAC header.

The Reply window 34 level sets the time offset for Tag reply (1 to K)and allows each Tag to choose one reply window out of a set of possiblereply windows.

The following is a numeric example that illustrates a non-limitingpossible practical implementation of the above multi-dimensional MACscheme:

Assume that 5 bits of the RANDOM_ID (N=32 groups) are used for “on”time, 8 bits for PHY addressing (M=256 groups), and 3 bits for the replywindow (K=8 groups). This divides the Tags into M*N−8,192 groups,therefore, per each broadcast message only approximately 1/8,000 of Tagswill pass the PHY screening and will turn their processor on. Only1/(M*N*K)=1/64,384 of Tags will reply in each reply window. Note thatwithout PIIY screening, 1/32 of tags would have had to decode the fullMAC message although the message is only relevant to 1/8,192 of tags.

It will be appreciated that such a MAC system can dynamically adjustitself to handle a smaller community of Tags by using a smaller numberof slots per frame (by non-limiting example 8 slots per frame), removingthe hardware addressing screening, and/or setting the number of replywindows to 1. This allows dividing the Tags into 8 groups, providing adynamic range of 8,000.

It should be noted that, in a case of a wrong estimation of Tagcommunity size, too many Tags may reply simultaneously causing uplinkcongestion. A congestion resolution algorithm is used to resolve suchcongestion and the results of the congestion resolution are used forre-estimation of the Tag community size and adjustments in the number ofTag sub-groups used.

High Reliability

When servicing a very large, unknown, community of Tags (i.e. X00,000),typical reliability of 99% or 99.9% will leave 1,000's of Tags not beingserviced. To address this problem, the MAC of the present invention isconfigured in the following manner.

When sending a broadcast message from one or more Readers 4 to a verylarge community of Tags 6, we would like each of the Tags 6 to replyonly once while overcoming possible collisions between Tag replies.

To significantly improve reliability of the system 2, a combination of“Session ID” (SID) and “relay” is used.

A typical broadcast 40 by a Reader 4 to a plurality of Tags 6 hasseveral steps:

-   -   1. A Reader 4 or plurality of Readers 4 transmits a broadcast 40        using a sub-grouping scheme relevant for the estimated Tag        community size, and the multiple access scheme capacity, the        Reader 4 will repeat transmission of the broadcast 40 enough        times to cover all tag sub-group combinations.    -   2. All broadcast 40 transmissions include an SID used for that        particular broadcast 40.    -   3. A Reader 4 receiving a Tag reply 42 will send an        acknowledgement message (Asks) 44 to the sending Tag 6 (see FIG.        4).    -   4. A Tag 6 receiving an acknowledgement 44 from a Reader 4 will        mark this SID as “acknowledged” and will not reply to further        broadcast messages 40 with the same SID.    -   5. The number of groups is adjusted based on the number of Tags        expected to reply to the next broadcast.    -   6. The SID may include a predefined length of time the SID is to        be stored by the Tag, thereby enabling reuse of the SID in        future broadcasts 40.    -   7. After covering all Tags sub-groups, the Reader 4 will repeat        the broadcast 40 to larger groups of Tags 6, and only those Tags        6 that have not received an acknowledgement 44 to their        broadcast reply 42, in which the acknowledgement 44 contains the        current SID, will reply.    -   8. After the process of step 6 is completed, the Reader 4 will        initiate a relay session (as illustrated in FIG. 5) to address        “Hidden Tags” 6 b.

An Implementation Example of the Above

Assume a single Reader 4, sending a broadcast message to 10,000 Tags 6.

Tags 6 are divided to 16 groups for the on-off timing. The multipleaccess scheme allows 10 Tags 6 to reply simultaneously with an 18%collision probability (unslotted aloha with 100 channels).

In order to have an average of 10 Tag replies, in a 10,000 Tagscommunity, the Tags 6 need to be divided to 1,000 groups. The divisionof groups is accomplished by a combination of 16 on-off groups, 8hardware addressing groups, and 8 reply windows, thereby dividing thetag community into 16*8=128 groups. The Reader 4 then sends thebroadcast message 40 to each of the 128 groups, each with 8 replywindows. This provides 128*8=1024 reply windows each of which canreceive 10 replies for a total of 10240 reply opportunities.

Therefore, the Reader 4 is expected to receive a reply 42 from10,000*82% of Tags 6, since there is an 18% expected reply collisionrate. Thereby leaving 1,800 Tags 6, on average, whose reply to theReader 4 has collided with other Tag replies and not been received by aReader 4.

The Reader 4 will then adjusts the number of groups from the previousbroadcast to accommodate an expected reply from the remaining 1,800 Tags6 by creating 16*2-32 groups, each with 6 reply windows, providing 1920reply opportunities. The 18% collision rate leaves 324 Tags 6, onaverage, whose reply has not been received by a Reader 4.

These 324 Tags 6 are now addressed by sending the broadcast 40 to16*1=16 groups with 2 reply windows, providing 320 reply opportunities.The 18% collision rate leaves 59 Tags 6 that are addressed by sendingthe broadcast 40 to 8*1=8 groups with 1 reply window per broadcastproviding 80 reply opportunities. This will result 7.5 replies pergroup, and collision rate of approximately 14%. Leaving 8 Tags 6 thatwill be addressed by sending the broadcast 40 to 4*1−4 groups with 1reply window per broadcast having an approximately 4% collision rate.This leaves 0.3 “unserviced Tags”.

That is to say, the total process included16*8*8+16*2*6+16*1*2+8*1*1+8*1*1−1,264 reply windows (average throughputof 10,000/1,264=7.9 replies per window), and results in an average of0.3 Tags 42 (99.997% reliability) that were not serviced by a Reader 4.

For comparison, a typical MAC system that does not use a SID, and Readerto Tag acknowledgement, should have repeated the first step (16*8 groupswith 8 reply windows) for 6 times (0.18⁶≈3*10⁻⁵)—requiring16*8*8*6=6,144 reply windows, 5 times longer than the method of thepresent invention.

Another alternative system would be to send 20 transmissions per replywindow, with a 33% collision rate. This process will need to be repeated10 times to get to the 99.997% reliability, but each “session” will onlyuse 4 reply windows resulting 16*8*4*10=5,120 windows, which is still 4times longer than the method of the present invention.

The Use of Relay for High Access Reliability

Due to the RF complexity of warehouses including, but not limited to,metal containers, concrete walls and metal shelves, and a limitedTag-Reader link budget, it is assumed that a Reader 4 will not be ableto achieve the required reliability of 99.9999%. Reliability may beincreased by increasing the number of Readers 4 being used, but this isgenerally not cost effective. A more cost effective method, according tothe teachings of the present invention, is to use any Tags 6 a presentwithin the warehouse in locations accessible to at least one of theReaders 4 as relays between Reader 4 and hidden Tags 6 b (tags that arenot accessed by the Reader directly).

After a Reader does not get any additional replies to its broadcasttransmission, or a rate of direct replies from the tags falls below apredetermined threshold, the reader will initiate a relay session. Thiswill be a specific message, marked as a “relay” message that “visible”Tags 6 a who receive it will re-transmit, acting as if they were Readers(i.e. transmitting a message in the same physical and logical parametersas Reader). Hidden Tags 6 b that do not receive the broadcast 40 fromthe Reader 4 due to link budget limitations, will receive at least oneof the relay messages sent by the visible Tags 6 a, and will send areply to the visible Tag 6 a. The visible Tag 6 a will forward thehidden Tag's reply to the Reader 4. Tag will consider itself as being ahidden Tag 6 b when it receives a forwarded message from visible Tag inwhich the message contains a new SID.

The Reader 4 may choose to broadcast the relay message to all Tags 6such that all visible Tags 6 a that receive the relay message willforward the message. Alternatively, Reader 4 may choose to broadcast therelay message only to predefined visible Tags 6 a such as, but notlimited to, visible Tags 6 a located in places in the warehouse, forexample, in proximity to areas known to have coverage problems or tovisible Tags 6 a having a stronger battery.

The number of relay “hops” is the minimal number of different Tags thatare needed in order to relay a message from the Reader 4 to a hidden Tag6 b and to relay the hidden Tag's 6 b reply back to the Reader 4 asillustrated in FIG. 5. Therefore, a relay may be limited to single hopwith one visible Tag between the Reader 4 and the hidden Tag 6 b.Additionally, a multi-hop method in which both the message andacknowledgments from the Reader 4 are forwarded by a visible Tag 6 a toa hidden Tag 6 b(1) that in turn forwards it until it is received by ahidden Tag 6 b(2) not in radio communication with either the Reader 4 orthe visible Tag 6 a. Likewise, the reply from the hidden Tag 6 b(2)forwarded by hidden Tag 6 b(1) to visible Tag 6 a and then to Reader 4is as illustrated in FIG. 6 which demonstrates a two hop data flow withacknowledgments between a Reader 4 and a hidden Tag 6 b(2).

The use of Relay for achieving high reliability is also applicable forunicast and multicast transmissions.

Acknowledge Optimization

In a system where downlink (Reader to Tag) speed is not significantlyfaster than uplink (Tag to Reader) speed, the acknowledgements maybecome a performance limiting factor. Therefore, the system of thepresent invention first aggregates a plurality of acks into a singlepacket so as to reduce overhead and to use Tag sub-addresses in the ack.The Tag sub-address may be added using other bits of the RANDOM_ID (thatis bits that are not used for other purposes such as on-off, PHYscreening, and reply window selection). Therefore, only Tags who repliedto the Reader's broadcast will look for their partial ID in the acks.Therefore, a shorter ID of 10-20 bits maybe used instead of the originalID of 32-64 bits, thereby improving downlink BW by a factor of up to 6.

A non-limiting example of Acknowledge optimization for a Tag ID of 8bytes, which equals 64 bits, according to the present invention is asfollows.

In a system according to the current state of the art before the presentinvention, for an average 10 replies per window, and 10 reply windows,100 acks should be sent, using 64*100-6.4 kbits (assuming negligible ackoverhead).

An exemplary embodiment of the present invention, may use 24 bits perack, where the 24 hits are taken from the non-used bits of theRANDOM_ID. This will use only 24*100=2.4 kbits.

Assuming no statistical dependency between RANDOM_ID bits used for theack and bits used for sub-grouping, the chances for a Tag receiving awrong ack when 100 Tags reply to a given broadcast is calculated asfollows. The chance of a Tag getting a wrong ack is 2⁻²⁴. The chance ofnone of the 100 Tags getting a wrong ack is 1−(1−2⁻²⁴)¹⁰⁰=6*10⁻⁶.

When the process repeats for approximately 100,000/80=1,250 times, theprobability of getting an error is 1−(1⁻⁶*10⁻⁶)¹²⁵⁰ which is less than1% after servicing 100,000 Tags.

Future Compatibility

Due to the dynamic nature of some applications in which the system ofthe present invention is used in which tagged items move into and out ofthe Tag community of the system at differing rates such as in, bynon-limiting example, warehouses, shipping yards and retail outlets, atany given time, the Tag community may include different generations ofTags, referred to herein as “old” and “new” Tags. It should be notedthat as used herein, the terms “old Tags” and “new Tags” refer to therelative generational difference of one Tag to another. Therefore, anygiven tag is considered “new” relative to Tags from older generationsand “old” in relative to Tags from newer generations.

Most communication systems currently in use with a non-symmetricarchitecture (i.e. typically having small number of sophisticatedReaders and large number of simple, low cost Tags), cope with Tag andReader evolution by implementing “Backward compatibility”. Two methodsare generally known in the prior art:

a) New generation Tags working with old generation Readers. This ishandled by new the generation Tags complying with the “old” rules, whichis the classically backward compatibility approach.

b) New generation Readers working with old generation Tags or with amixed network of old and new Tags. This is typically done by eitherhaving new generation Reader working in old generation protocol in amixed network, or new generation Reader having to maintain a Taggeneration association table, and maintaining different protocols foruse with different Tag generations.

The system of the present invention provides a unique “forwardcompatibility” method. Herebelow are two non-limiting examples.

End Point MAC Forward Compatibility

MAC messages contain a MAC length indicator indicating the MAC payload(e.g. application layer data) beginning. The MAC itself is processedaccording to field content. Therefore new generation Tags or Readers mayadd new fields to the MAC. Old generation Tags/Readers stop decoding theMAC when they reach unfamiliar information and “jump” to the payloadstart, while new generation Tags will continue to process the remainderof the MAC. The same method can be used when an old generation Readerdecodes a new generation Tag message. That is to say, each generation ofReaders and/or Tags decode that portion of the Media Access Controlprotocol with which it is familiar, ignores that portion of the MediaAccess Control protocol with which it is unfamiliar and jumps to thepayload start point. In this way there is no need to send differentmessages to different generation Tags, so one broadcast message can holdboth old generation Tags information and instructions with additionalnew generation Tag information, thereby increasing system efficiency.

Mesh MAC Forward Compatibility

In a mesh network that has a small number of Readers, and large numberof Tags, where Tags can communicate with each other, usually acting asrelays, forwarding Reader message to remote Tags, or returning remoteTag reply, a mixture of Tags generations or versions is morechallenging.

The present invention adds a “future block” to the MAC data relayed fromone Tag to the other. Old generation Tags will forward this block as is,while new generation Tags/Readers can decode the content of this block,and change it if applicable. That is to say, the Media Access Controlprotocol includes a future block that old generation Tags forwardwithout decoding, and new generation tag Readers and/or Tags decode asneeded. This allows for efficient mixture of Tags and Readers ofdifferent versions allowing old generation Tags to relay new generationmessages, new generation Tags to relay messages to old generation tagswhile avoiding the for Readers to send different broadcasts for eachgeneration of Tags in the Tag community.

It will be appreciated that the above descriptions are intended only toserve as examples and that many other embodiments are possible withinthe spirit and the scope of the present invention.

1. A system of radio frequency communication between Readers and Tags,the system comprising; (a) at least one tag Reader; (b) a plurality ofTags deployed in a region in which at least some of the Tags are inradio communication with said tag Reader, and each Tag in the pluralityof Tags is associated with a unique tag ID, and (c) a Media AccessControl protocol according to which: said tag Reader is configured tosend and receive radio communications to and from at least some of saidplurality of Tags, said plurality of Tags are configured to send andreceive radio communications to and from said tag Reader, and said MediaAccess Control protocol is configured to control said radiocommunications between said tag Reader and said plurality of Tags, andsaid Media Access Control protocol includes a multi-dimensionaladdressing scheme, said multi-dimensional addressing scheme using a hashfunction to transform said tag ID into a pseudo random ID and enablingdivision of said plurality of Tags into one or more groups for differentstages of Media Access Control protocol processing, based on differentportions of said tag pseudo random ID.
 2. (canceled)
 3. The system ofclaim 21, wherein a portion of said tag pseudo random ID is used formessage screening in the PHY level and thereby avoid a need for at leastone tag in said plurality of Tags to fully process a Media AccessControl protocol portion of said radio communication.
 4. The system ofclaim 1, wherein a portion of said tag pseudo random ID is used forsetting the on/off timing of at least one tag in said plurality of Tags.5. The system of claim 1, wherein a portion of the ID is used by atleast one tag to choose one reply window out of a set of possible replywindows.
 6. The system of claim 1, wherein a portion of the ID is usedby said tag Reader as an acknowledgement to at least one Tag of saidplurality of Tags that said tag Reader received a radio communicationfrom said at least one Tag.
 7. A system of radio frequency communicationbetween Readers and Tags, the system comprising; (a) at least one tagReader; (b) a plurality of Tags deployed in a region in which at leastsome of the Tags are in radio communication with said tag Reader, atleast some tags are in radio communication with other Tags in saidplurality of Tags and each Tag in the plurality of Tags is associatedwith a unique tag ID, and (c) a Media Access Control protocol accordingto which: said tag Reader is configured to send and receive radiocommunications to and from at least some of said plurality of Tags, saidplurality of Tags are configured to send and receive radiocommunications to and from said tag Reader and other Tags in saidplurality of Tags, wherein the plurality of Tags are assigned apseudo-random ID based on the unique tag ID and a hash function and theplurality of Tags are divided into one or more groups based at least onone or more parts of the pseudo-random ID, and said Media Access Controlprotocol is configured to control said radio communications between saidtag Reader and said plurality of Tags, such that said Media AccessControl protocol uses a combination of a session ID, acknowledgementsand use of other tags as relays to achieve high reliability of radiocommunication, in broadcast, multicast and unicast modes.
 8. The systemof claim 7, wherein a portion of said tag ID is used by said tag Readerto indicate to at least one tag of said plurality of Tags that said tagReader received a said radio communication from that particular tag. 9.The system of claim 7, wherein said tag Reader initiates a relay sessionafter a rate of direct replies from said plurality of tags falls below apredetermined threshold.
 10. The system of claim 7, wherein eachbroadcast message is given a session ID, and at least one tag of saidplurality of tags replies to radio communications having a particularsession ID until said at least one tag receives an acknowledgement radiocommunication having the same session ID.
 11. A system of radiofrequency communication between Readers and Tags, the system comprising;(a) at least one tag Reader; (b) a plurality of Tags deployed in aregion in which at least some of the Tags are in radio communicationwith said tag Reader, and wherein at least some tags are in radiocommunication with other Tags in said plurality of Tags, and each Tag inthe plurality of Tags is associated with a unique tag ID; and (c) aMedia Access Control protocol according to which: said tag Reader isconfigured to send and receive radio communications to and from at leastsome of said plurality of Tags, said plurality of Tags are configured tosend and receive radio communications to and from said tag Reader andother Tags in said plurality of Tags, and said Media Access Controlprotocol is configured to control said radio communication between saidtag Reader and said plurality of Tags, wherein the plurality of Tags areassigned a pseudo-random ID based on the unique to ID and a hashfunction and the plurality of Tags are divided into one or more groupsbased at least on one or more parts of the pseudo-random ID; and whereinsaid Media Access Control protocol includes tag reply parameters in eachsaid radio communication so as to enable tags to respond to said tagReader in a manner that is compatible with that tag Reader'scapabilities.
 12. The system of claim 11, wherein said radiocommunications includes an indication of a payload starting point, andeach generation of tag Readers and/or Tags decode that portion of saidMedia Access Control protocol with which it is familiar, ignores thatportion of said Media Access Control protocol with which it isunfamiliar and jumps to said payload start point, and said radiocommunication includes a block of variable length, such that a firstgeneration Tags is configured to forward said block withoutmodification, and a second generation tag Reader and/or Tag isconfigured to decode and modify said block as needed.
 13. A system ofradio frequency communication between Readers and Tags, the systemcomprising; (a) at least one tag Reader; (b) a plurality of Tagsdeployed in a region in which at least some of the Tags are in radiocommunication with said tag Reader, and each Tag in the plurality ofTags is associated with a unique tag ID, and (c) a Media Access Controlprotocol according to which: said tag Reader is configured to send andreceive radio communications to and from at least some of said pluralityof Tags, said plurality of Tags are configured to send and receive radiocommunications to and from said tag Reader, wherein the plurality ofTags are assigned a pseudo-random ID based on the unique tag ID and ahash function and the plurality of Tags are divided into one or moregroups based at least on one or more parts of the pseudo-random ID, andsaid Media Access Control protocol is configured to control said radiocommunication between said tag Reader and said plurality of Tags;wherein said radio communication includes an indication of a payloadstart point, and each generation of tag Readers and/or Tags isconfigured to decode that portion of said Media Access Control protocolwith which it is familiar, ignores that portion of said Media AccessControl protocol with which it is unfamiliar, and jumps to said payloadstart point, allowing tag Readers to efficiently communicate with amixture of tag generations and/or tag versions.
 14. A system of radiofrequency communication between Readers and Tags, the system comprising;(a) at least one tag Reader; (b) a plurality of Tags deployed in aregion in which at least some of the Tags are in radio communicationwith said tag Reader, at least some tags are in radio communication withother Tags in said plurality of Tags, and each Tag in the plurality ofTags is associated with a unique tag ID; and (c) a Media Access Controlprotocol according to which: said tag Reader is configured to send andreceive radio communication to and from at least some of said pluralityof Tags, said plurality of Tags are configured to send and receive radiocommunication to and from said tag Reader and other Tags in saidplurality of Tags, wherein the plurality of Tags are assigned apseudo-random ID based on the unique tag ID and a hash function and theplurality of Tags are divided into one or more groups based at least onone or more parts of the pseudo-random ID, and said Media Access Controlprotocol is configured to control said radio communication between saidtag Reader and said plurality of Tags; wherein said radio communicationincludes a block of variable length, wherein a first generation Tags isconfigured to forward said block without modification, and a secondgeneration tag Reader and/or Tag is configured to decode and modify saidblock as needed.
 15. The system of claim 14, wherein said radiocommunication includes an indication of payload start point, and eachgeneration of tag Readers and/or Tags decode that portion of said MediaAccess Control protocol with which it is familiar, ignores that portionof said Media Access Control protocol with which it is unfamiliar, andjumps to said payload start point, allowing tag Readers to efficientlycommunicate with a mixture of tag generations and/or tag versions.